Description

This thesis focuses on microchannel heat exchangers, and the methods used to try and reduce the charge
contained in them. First, four condensers were designed and manufactured with the same air side characteristics and
same tube design (with the exception of some blocked ports in one design) so that any difference in charge, capacity,
pressure drop etc. can be attributed to flow geometry. These designs were a two circuit serpentine design, a two
pass parallel flow design, and two one pass parallel flow designs. The difference between the two one pass designs
were the number of microchannel ports in the tubes, one with 19 ports and the other with 10.
The one pass design with 10 ports contained the least amount of charge, on average about 20 g, while the
other one pass design contained the most charge, about 28 g. The reason for this difference was not only because
the smaller tube volume, but an increase in mass flux also decrease the amount of charge in the exit header by 20-
30%. The serpentine condenser also had very low charge amounts, but pressure drop was about ten times higher
than in the parallel flow designs. This caused a 4% lower COP than what was given with the one pass design.
Two serpentine evaporators have also been examined. The evaporators had similar core volumes of about
870 cm3. One evaporator had double the fin length by using a “splitter” fin, which reduced the length of the
evaporator tube and therefore reduced the internal volume. Both evaporators performed almost exactly the same,
but the splitter fin evaporator had less of a pressure drop and contained 15% less charge.
Finally, the models developed for this thesis were used to theoretically compare the performance and
charge of six different refrigerants, propane (which was the refrigerant used for experiments), R22, R134a,
isobutane, ammonia, and R410A in a condenser. A pressure drop based on a 1% decrease of Carnot COP was found
for each refrigerant. Ammonia was found to be able to have same capacity as the other refrigerants, but able to
achieve the capacity with a much smaller cross sectional area and a very small amount of charge.

Issue Date:

2005-06

Publisher:

Air Conditioning and Refrigeration Center. College of Engineering. University of Illinois at Urbana-Champaign.